1. Field
The present invention relates to a capacitive transducer, and more specifically relates to a capacitive acoustic sensor.
2. Related Art
One type of capacitive transducer is an acoustic sensor that may be used in a microphone. The acoustic sensor is constructed from a diaphragm (an oscillating electrode plate), and a fixed electrode plate arranged opposite each other with a tiny gap (space) interposed therebetween. The diaphragm is formed from a thin-film about 1 μm thick; therefore, the diaphragm will make minute oscillations in response to the acoustic pressure impinging thereon. Acoustic vibrations may be detected by detecting the change in capacitance when the diaphragm vibrates because the oscillation of the oscillating electrode plate changes the size of the gap between the oscillating electrode plate and the opposing electrode.
For an acoustic sensor constructed in this manner, when the diaphragm 11 flexes greatly and comes in contact with the fixed electrode plate 12, the diaphragm adheres to the fixed electrode plate 12 and no longer returns to its original position as exemplified in FIG. 1A. This phenomenon is called “sticking”. This sticking of the diaphragm may be caused when, for example, a large sound wave impinges on the diaphragm; a large amount of compressed air (air pressure) impinges on the diaphragm in an acoustic sensor during a drop test; or air is drawn forcefully into the acoustic sensor. The sticking may also occur while manufacturing the acoustic sensor.
When the diaphragm sticks to the fixed electrode plate in this manner, this inhibits the vibration of the diaphragm, and thus the acoustic sensor is no longer capable of detecting acoustic vibrations. Therefore, it is necessary to prevent this kind of sticking. Japanese Unexamined Patent Application Publication No. 2006-157863 discloses an acoustic sensor for addressing this issue. The acoustic sensor is provided with a plurality of stoppers, which are bumps on the surface of the fixed electrode facing the diaphragm. In general the stoppers are provided spaced equally over the entire surface of the fixed electrode plate; if the stoppers are made as thin as possible, then the contact surface area between the diaphragm and the fixed electrode plate (or, the stopper) is reduced, and the diaphragm tends not to stick to the fixed electrode plate.
However, in order to provide a stopper in the acoustic sensor so that the diaphragm does not stick, it is necessary to adjust the spacing between stoppers. FIG. 1B to FIG. 1D schematically illustrate the state of the diaphragm 11 in cases where the spacing between stoppers 13 is too large, is suitable, and is too small respectively. In FIG. 1C the spacing D between stoppers 13 is suitable. In this case, even if the diaphragm 11 adheres to the fixed electrode plate 12, as illustrated by the double-dotted dash lines in FIG. 1C, the diaphragm tends not to stick because the contacts surface area between the stoppers 13 and the diaphragm 11 is small; further, as illustrated by the solid line in FIG. 1C, the diaphragm 11 returns to its original position using its own elastic restorative force.
In contrast, as illustrated in FIG. 1B, the spacing d between the stoppers 13 is narrower than when the spacing is suitable; in this case, even if the stoppers 13 are thin and the surface area of the tips are small, there is a limit to the minimization of the tip end surface of the stoppers, and therefore the total surface area of the tip end surface comprising the overall stoppers becomes large and as a result, in this case, the diaphragm 11 sticks to the tip end surfaces of the stoppers 13 spanning a almost the entire surface or a wide region, and thus the diaphragm 11 sticks to the stoppers 13.
As illustrated in FIG. 1D, when the spacing d between stoppers 13 is wider than the suitable spacing, even if the diaphragm 11 comes in contact with the stoppers 13, a portion of the diaphragm 11 falls through between adjacent stoppers 13 and comes in contact with the fixed electrode plate 12. When the diaphragm 11 sticks to the fixed electrode plate 12 in this manner, even if there is only one contact portion, the contact surface area tends to be much larger than the tip end surface area of the stoppers 13, and therefore the diaphragm 11 sticks to the fixed electrode 12.
As a result, a conventional acoustic sensor is prone to sticking whether the spacing between stoppers is too large or too small, and thus it is necessary to provide stoppers 13 so that a suitable spacing is formed.
Moreover, the length of the stoppers (the protruding length), influences the tendency for the diaphragm to break (anti-breaking property), and the tendency for the diaphragm to stick (anti-sticking property). This influence will be described using FIG. 2. In the acoustic sensor illustrated in FIG. 2, stoppers 13 protrude from a back plate 14 supporting the fixed electrode 12.
If the stoppers are short, as illustrated in FIG. 2A, the diaphragm 11 deforms to a large extent, because there is wide spacing between the undeformed diaphragm 11 and the stoppers 13. Therefore, the diaphragm 11 acquires a large elastic restorative force when the diaphragm abuts the stopper 13, and the diaphragm 11 tends not to stick. However, when the stoppers 13 are short, if the diaphragm 11 is subject to an excessive amount of pressure the diaphragm 11 deforms to a large extent and tends to break, because there is wide spacing between the diaphragm 11 and the stoppers 13. Particularly the sections near where the diaphragm 11 is fixed tend to break when there is a large deformation of the diaphragm 11.
If the stoppers are long, as illustrated in FIG. 2B, the maximum displacement of the diaphragm 11 is small because of the narrow spacing between the undeformed diaphragm 11 and the stopper 13, and the diaphragm 11 tends not to break. However, when the stoppers are long, the diaphragm 11 acquires a small elastic restorative force when the diaphragm 11 abuts the stoppers 13 and tends to stick because there is a narrow spacing between the diaphragm 11 and the stoppers 13.
Consequently, stoppers constructed in the conventional manner provide a trade-off between the desire to improve the anti-sticking and the anti-breaking properties of the diaphragm even when there is a suitable spacing between stoppers.